Hi vMix and fellow forum members,

I had a live stream yesterday for a friend that was formatted in an interview/Q&A style. During the live stream, the audio was mostly fine for the most part.
However, when I attempted to play a soft audio bgm in the background, I noticed that our guest speaker's audio quality would drop and she would begin to sound
like she was underwater/garbled. When I stopped the audio it would return to normal and she would sound fine again.

Also, during the live stream, I noticed that I'd drop a fairly large amount of frames for the recording. If anyone could recommend a quality recording setting
based on my PC specs listed below that would be great help. I've tried the recommended settings listed in the vMix user guide but my friend I'm recording for
requested it be in MP4 format.

Wondering if someone who experienced similar things could point me in the right direction to get this resolved. See below for full specs:

Internet: 1G up & down
Mobo: Asus ROG Maximus XI Formula LGA 1151
Processor: Intel i9-9900K @ 3.60GHz
RAM: 64 GB @ 3200 mhz
GPU: Nvidia RTX 2080 Ti 11GB (model #11G-P4-2487-KR)

vMix preset: 1080P @59.94

vMix audio config:

Guest: Audio Source-BUS A, mic input sent to A,B,C,D
Host: Audio Source-BUS B, mic input sent to A,C,D
Production Member: Audio Source-BUS C, mic sent to B,D
myself soloing on Audio BUS D, sent my mic to B,C only

Recording configuration:
Multicorder:
MP4 Video Bitrate @ 75 H264Main, Audio Bitrate @ 192, Hardware Encoder Selected, outputs 1,2,3 selected

Stream settings:
Bitrate: 6000
Resolution: 1080p
audio: 128
profile: main
level: 3.1
preset: veryfast
audio: AAC
keyframe: 2
threads: 4
network buffer: 20
strict CBR enabled
application: FFMPEG, hardware encoder selected

The stream was sent to YT.

some short notes:
1)Guest was hardwired into their device used during the interview/Q&A call.
2)the music I would feed into both the host and the guest's audio bus (A&B), that's when the issues would happen (they wanted to hear the music)
3) I was sending all vMix call guests (the three people listed above) 720P @ 2000 kbps - advanced call settings were default, and remote guest video bandwidth was auto

Any help would be appreciated as this will be a reoccurring event on a bi-monthly basis.